You need to make sure you're also backing up the database. Usually this is done with mysqldump to save an SQL file to some place within the directories that you're backing up already.

As for Piwigo data, (my understanding is that) one only needs to backup the 'upload' and 'local' directories.

I can't find a page in the documentation wiki about backing up...

Yep, that sounds correct. I'm not sure about plugins: I don't *think* they're supposed to store anything anywhere else, but I did read somewhere about a plugin storing things in _data (which is usually only for transient, re-creatable, generated things), but I guess it'd be hard to know which plugins do it and which don't.

I'm doing exactly what you say (via my backup script) and so far as I can tell it works perfectly.

Thanks for the link. It seems that there is no way to backup plugins.
I wonder though, if I backup the entire "/" can I just restore the whole thing (after installing a fresh Piwigo – same version) by copying the whole /var/www/piwigo directory?